Wave signaling system



Oct. 27, 1942. R. M. sPRAGuE l 2,299,937

` WAVE SIGNALING SYSTEM i Filed oct. 27, 1959 SSheQtS-Sht 1 LOW'- PASSFILTER' SIGNHL. GENERATOR 4 Oct. 27, l942 R. M. SPRAGUE WAVE SIGALINGSYSTEM 5 Sheets--SheeilI 2 Filed oct. 27, 1939 ril .wrLl

INV'ENTOR ,M..

ATTORNE oct; 27, 1942.

R:A M. PRAGUE WAVE SIGNALING SYSTEM- `Filed Oct. 274I 1939 5Slleells-Shee'l'l 4 fi w . www

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oct- 27, 1942- R. M. SPRAGUE WAVE SIGNALING SYSTEM Filed oct. 27, 19:59v5 sheets-sheet 5 wfg-:N R

BY ATTORNEY Patented Oct. 27, 1942 WAVE SIGNALING SYSTEM Robert M.Sprague, Garden City, N. Y., assgnor to Press Wireless, Inc., Chicago,Ill., a corporation of Delaware 'Application October 27, 1939, SerialNo. 301,563

14 Claims. (Cl. TIS- 6.7)

This invention relates to wave signaling systems and more especially tosystems of carrier wave transmission wherein the effects of parasitics,noise, harmonic distortion fading and undesirable level variations areto be avoided.

A principal object of the invention relates to a system for transmittingsignals whether oral, visual or control, through the intermediary of atransmission link, for example a high-frequency channel or radiochannel, wherein the signal reproduction is rendered substantiallyindependent of undesirable level changes.

AAnother principal object is to provide an improved r'adio signalingsystem which is singularly free from the effects of fading, echo,harmonic distortion, parasitic noises'and the like.

Another principal object is to provide an improved system of secrecysignaling` over a carrier channel wherein parasitic disturbancesincluding level distortion or fading are materially reduced.

A further object is to provide an improved system of transmittingmessage signals over a carrier or high frequency channel by frequencymodulation.

A feature of the invention relates to a novel A further feature relatesto a frequency modulating system employing a photo-electric cell orsimilar device as the source of modulating voltage, and wherein afrequency modulated oscillator is employed and so arranged that theoscillator output is substantially free from amplitude variations offrequency components of the said voltage source.

, A further feature relates to a receiver or reproducer of facsimilesignals and the like, employing a specially designed band-pass filterfor eliminating harmonic distortion in the transmitting medium or linkand also for eliminating noise components below the actual frequencyspectrum employed for signaling.

A further feature relates to a facsimile receiver or the like for use ina frequency modulation system and having an automatic level control'determined by the received side-bands as dismethod of convertingamplitude variations into corresponding frequency variations vofsubstantially uniform amplitude.

Another feature relates to a frequency modulation system employing apair of oscillators for f producing a variable beat frequency whereinthe beat frequency is varied in accordance with capacitychangescontrolled by a grid-controlled electron tube. As a result of thisfeature oscillators can be employed whose frequency is independent ofvoltages on their elements thus giving a beat frequency oscillator thatis substantially independent of power supply voltage variations on theoscillators.

of the band-pass type. Preferably, in accordance y with the invention,the saidl lower limit is also chosen so that unattenuated frequencyshifts are l obtained up to the limits of the filter.

A further feature relates to an improved frequency modulator wherein alinear variationI of output frequency with relation to input voltage isobtained over a Very wide range.

tinguished from the carrier.

A further feature relates to a receiver for frequency modulation systemsemploying the combination of a specially designed filter and powerlimiter whereby all undesirable level variations which are not fullycompensated for by the automatic volume -control are ironed out in'orderto supply an undistorted signal of unvarying level to succeedingcircuits in the receiver.

A further feature relates to a frequency demodujlator or converterarrangement for a receiver operated by a received frequency modulatedwave, whereby the frequency modulations are converted to amplitudevariations with a linear ratio between the frequency and amplitude andwith a high ratio of signal-to-noise.

A further feature relates to a transmitter of the frequency-modulatedwave type wherein novel means are provided for adjusting the frequencybands in accordance with the minimum and maximum amplitudes of thesignals to be transmitted. Thus in the' case of a facsimile transmitter,it is possible conveniently and expeditiously to obtain the zero orminimum frequency-modulating Voltage which is correlated to -l zerooutput voltage from the facsimile generator. A still further featurerelates to facsimile transmitting and receiving systems wherein thepicture signals are transmitted by frequency modenumerated will beapparent after a considera` While the invention will be illustrated inconnection with facsimile transmitting and receiving systems, it will beunderstood that this is merely `for explanatory purposes and thatvarious quency-to-amplitude conversion section of Fig. 4.

Fig. 5 is a layout of Figs. l to 4 of the drawings. Fig. 6 is amodication of the amplitude-tofrequency conversion section of Fig. 2.

The transmitter Referring to Fig. 1, block T represents any well-knownform of signal generator, for example a facsimile converter of the typevshown in application Serial No. 213,584, filed June 14, 1938.Preferably the device T is of a known type whereby the signals to betransmitted are generated in the form of a carrier of relatively low oraudio frequency, for example 1800 C. P. S. and having amplitudemodulations or variations corresponding to the original signals to betransmitted. It will be understood that the term carrier is used in itsbroad aspect as including either a sustained alternating current, apulsating current or a chopped current such as that produced for exampleby a light chopper photo-electric cell combination as is well-known inthe facsimile art. f

The signal from device T is applied across the input terminals A, B, andthence through the network comprising resistor elements I to 5 inclusiveand thencethrough input transformer 6. The output of transformer 6 isfed to any wellthrough potentiometer 1. Merely for purposes ofillustration, this amplifier comprises a three element tube 8 connectedinto circuit inA the usual way and including the grid-bias resistor 9;the

plate resistors II, I6, through which plate voltage is fed over theconductor C from the A. C.

' power supply, the latter consisting for example switch I3 to thecontrol-grid 28a of tube 28.v

Tube 28 may be of any well-known type, for example, a type 6C8-Gcomprising two sets of triode elements with the cathodes 28h and 28econnected together and biassed with respect to the grids 28a and 28d bythe bias resistor 29. The plates 28e and 28j are connected to the platesupply conductor C through the limiting resistor 33 and the respectivecoupling resistors 38, 3|, so as to provide a balanced input to thesucceeding amplifier stage comprising amplifier tubes 4| and 42. to tube28 through the condensers 34 and 35 in balanced or push-pull relation,the tubes 4| and 42 being preferably of the pentode type. The

The tubes 4I and 42 are coupled'y Aknown form of linear amplifierpreferably tion of the following detailed descriptions and the appendedclaims.

plates of tubes 4| and 42 are connected to the D. C. supply through theprimary winding of coupling transformer v48. Preferably, although notnecessarily, transformer 48 is provided with an auxiliary secondary orfeed-back winding D which is connected through resistor I1 to thecathode of tube 8 to reduce distortion in the amplifier. The mainsecondary of transformer o43 is applied to a rectifier 44 and thencethrough the low-pass nlter 45 to eliminate the 1800 cycle carriercomponent. Consequently at the output terminals of lter 45 there ispresent only the amplitude demodulated signal corresponding to thephoto-electric current delivered by the photoelectric cell of machine T.

The frequency inverter or modulator The demodulated and amplified signalvoltages developed across resistor 41 are then fed through the isolatingresistor 48 to the control grid 50a o a pentode tube 58 which, in'accordance with the invention, is employed as a modulator tube tocontrol the beat frequency developed by the oscillator tubes and 85.Tube 85 is provided with a 'variable frequency control circuit whiletube 86 'is of a fixed frequency which is determined -by the tunedoscillatory circuit 15, 16, connected between the control grid 88a andthe cathodes 86h, 86C, through respective bias resistors |28, |21, andthrough by-pass condensers |30, |3I. The plate 86d is coupled back tothe control grid 86a through the feed-back condenser 89 and a grid leakcondenser combination 81, 88. Likewise plate 88e is coupled to controlgrid 86f through condenser 85. As a result of these connections,oscillations cf xed frequency, for example 600 kilocycles are developedacross resistor 82. The tube 85 is similarly provided with circuitconnections including elements 62, 63, 64, 66, 81, |24, |25, |28 and|29, and a tunable oscillatory circuit comprising coil 8| andshuntcondensers 59, 68, |23, whereby sustained oscillations of thedesired range, lfor example 601.8 to 603 kilocycles are developed acrossresistor 12. Oscillator tube 65 is adjusted to the correct frequency,for example the lower limit of 601.8 kilocycles by variable condensers58 and 60 and the frequency of this oscillator tube is automaticallyvaried in accordance with a change in the plate resistance orplate-to-cathode capacity of tube 50.

This frequency modulation is accomplished in the following manner. Theplate circuit of tube 50 is connected across the oscillatory circuit 59,60, 6|, through series condenser 54. Tube 50 when connected as shown, iscapable of varying its plate resistance and to a certain extenty itsplate-to-cathode capacitance in accordance with the control grid voltageimpressed thereon, and since this plate resistance is in series with condenser 54 any change in said plate resistance has the effect of changingthe effective capacity of condenser 54 so far as affecting the frequencyof the oscillatory circuit is concerned and hence the frequency of thevoltages developed across resistor 12. Preferably tube 58 is of thepentode type, for example a type GF6, which after numerous tests I havefound appears to change the output frequency of tube 65 substantiallylinearly over the widest frequency range. The suppressor-grid 5817 oftube 50 is connected to cathode 50c which latter is biassed negativelywith respect to control-grid 58a by the bias resistor 5l and itsb-y-pass condenser 52. Preferably, plate 50d and screen-grid 50e areconnected totube 65, very little change modulation frequencies appliedto .transmitter gether and to the plate supply conductor E' throughresistors 53 and 51 and the filter choke |22 which latter is by-passedby the condenser 58.

I have also found 'that by using the varied plate resistance or thevaried plate-to-cathode capacitance of tube 50 to vary the frequency ofin the amplitude of the resultant beat frequency between tubes 65 and 86is produced. Consequently the beat frequency of tubes 65 and 86 issubstantially undisturbed by any frequency component that may exist inthe photo-electric cell output voltage ol machine T. f A

.The beat -frequency from tubes 65 and 86 is obtained by applying theirrespective outputs to respective signal grids of the mixer tube 60condenserl |32 is also provided, and by means of condenser 93 and highfrequency choke coil 94,

the output currents flowing through coupling resistor 99 are the desiredaudio frequency signals which vary in frequency for example between 1800C. P. S. and 3000C. P. S. in accordance with the corresponding shadevalues of the particular area of the subject matter in' machine T whichis being momentarily scanned. The D. C. supply for tube 90 is appliedover conductor F and thence through resistors and 91, the voltage forscreen-grid 90e also being applied over conductor F in series withresistor 90, it being understoodthat the resistors 08 and |00 areby-passed by respective condensers 96, |0| and |02.

The beat or audio frequencies flowing in the plate circuit of tube 90are impressed upon the ,control-grid of a suitable power amplifier tube|03, the grid of which is biassed by resistor |04 and by-pass condenser|05. The amplied output of tube |03 is applied through audio frequencytransformer |06 to a low-pass lter |01 RT, there is provided a tubewhich is a monitor mixing tube to combine apart ofthe 1800 cyclesub-carrier signal from machine T with the output frequency from tube|03. `If as above assumed, the picture shades are represented by afrequency range between 1800 and 3000 C. P. S., with 1800 C. P. S.representing black andv 3000 C. P. S. representing white, it isnecessary to adjust the output frequency from tube |03 to 1800 cycleswhen zero or a predetermined minimum-voltage is impressed on the grid oftube 50.. The, 1800 cycle signal which is appliedto the tube 20 directlyfrom the output of tube 8 is used as a standard of comparison. For thispurpose, tube 20 is preferably of the GFS-G type comprising two sets oftriode elements. The grid 20a of lonetriode is excited by a part of the1800 cycle signal from the output of tube 8. The grid 20h of the othertriode is excited by part of the output of tube |03. The plates 20c and20d are supplied with D. C. plate potential respectively over conductorsC and E; Likewise plates 20c and 20d, are connected through asingle-pole double-throw switch 23 to amonitoring jack 24 into which afrequency meter or a pair of head phones may be plugged. Switch 23 ispreferably of the type such that in its extreme end positions it makescontact with the respective xed in order to eliminate any harmonicdistortion.

Filter |01 is designed to Acut-off frequencies above the first octave ofthe lowest signal frequency.

- The filtered signals are then fed through the resistor network or pad|06 to ||2 to any suitable form of radio transmitter representedschematically by the block RT.

Preferably, and in accordance with the invention, the system is soadjusted that for the maximum range of shade values in the subjectmatter being scanned by the machine T, for example that corresponding toblack and white respectively, the signal frequenciesl are confined toone octave or less. -Thus, if a black area isv represented by afrequency of 1800 C. P. S., then a white area should be represented by afrequency of less than 3600 C. P. S., for example` 3000 C. P. S.` sinceduring transmission the second harmonic often appears with an amplitudemany times that of the desired fundamental. If the signal frequenciesare restricted below the second harmonic of the lowest signal frequencyas will be described hereinbelow, then by` means of lter |01 the secondharmonic distortion during transmission is effectively eliminated so faras any effect on the reproduced signal is concerned.

In order to be able to correlate the upper and lower shade values of thesubject matter with the corresponding upper and lower limits of thecontacts, but in an -intermediate position it makes contact with bothxed contacts. When the blade of switch 23 is in its intermediateposition as shown in the drawing, the 1800 C. P. S. signal from machineT and the signal from tube |03 are both applied to jack 24 forcomparison purposes. When the blade of switch 23 is in its right-handposition, only the signal from tube |03 is applied to the jack. Asingle-pole double-throw switch I3 is provided between the tubes 8 and28, and when the blade of this switch is in its uppermost position,direct ground is applied to the control grid of tube 28. When the bladeis in its lowermost position, the switch I3 connects the output of tube8 through potentiometer I4 to tube 28. By throwing the blade of switchI3 to its uppermost position, no

voltage exists at grid a of modulator tube 50.

A normal white signal is then sent; from machine T and is heard at jack24 with switch 25 in its intermediate position which combines input andoutput frequencies. The beat frequency of tubes and 66 is thereadjusted, for example by condenser 59 until the output from tube |03appearing at jack 24 is the same as that derived from machine T throughtube 8. The switch |3 is then thrown to its lowermost position wherebyvtheamplified picture signal voltages from ma- The receiver Referringvto Figs. 3, 4, a description will now be given of a receivingarrangement that may be used. The signal from the radio transmitter RTof Fig. 1 is applied to any well-known form of radio receiverrepresented `by the block RR, wherein the modulations of from 1800 to3000 cycles are detected or demodulated. These demodula-ted signals arethen applied to the resistor pad comprising resistor elements to 205inclusive and thence through the band-pass filter 206. Filter 206eliminates all harmonics of the signal frequencies of the 1800-3000cycle band that may have appeared as a result of selective fading orother causes during transmission, and the said filter passes only thesignal as actually passed by the filter' |01 (Fig. l). Filter 206 alsoeliminates any noise voltage outside its band-pass. Since there may beside-band frequencies present because of the variation of the carrierduring transmission, the lower cut-off frequency of filter 206 must notbe chosen too close to the lower end of the actual signal band, that is,too close to 1800 C. P. S. nor to the upper limit of this band, -thatis, 3000 C. P. S. Preferably, the lower cut-off of filter 206 is spacedfrom the lower limit of the signal band the same distance as the spacingbetween the upper cut-off of filter 206 and the upper limit of thesignal frequency band. Thus the pass-band is symmetrical and issuiiicient to pass all desired side-band frequencies. tween 1500 and3300 cycles.

The signal from filter 206 may vary in amplitude and must therefore beironed out to a uni' form level. This is effected by the A. V. C.circuit of amplier tubes 236 and and by the power limiter tube 21|. Tube2|1 is a straight amplifier tube preferably o'f the double triode typehaving two separate output circuits, one of these output circuitsincluding plate 2|1a and cathode 2|1b leads to the monitoring meter 226and monitoring jack 245, whereby the averagelevel of the signal can bedetermined and adjusted by potentiometer 2|3. The other output circuitincluding plate 2|1c and cathode 2|1d feeds into amplifier tubes 236 and25|. Tubes 236 and 25| are preferably high gain pentodes having a remotecut-off. The D. C. plate supply for tube 2|1 is derived from a suitableA. C. source 342 over power transformer 333, full-wave rectifier 334, D.C. smoothing lter 335, 336, 331, conductor G, thence through the low`frequency choke 290, conductor Gi and resistor 233 to plate 2|1c. Platesupply conductor GI is also connected by way of resistors 221 `and 222to plate 2|1a the various resistors being provided with suitable bypasscondensers as shown. The signal developed across coupling resistor 229is impressed through condenser 230 on the control-grid 236a of tube 236,the cathode 23617 of which is connected to the suppressor-grid 236,0.The plate 236d and screen-grid 236e are supplied with appropriate D.A C.potentials from conductors G, GI. The outputfof tube 236 is fed to asimilar amplifier pentode 25|. The output of tube 25| is coupled throughresistor 255 and condenser 256 to the connected control grids 264a, 264bof the double triode tube 264. The signal current in the circuit ofplate 264C is applied to the rectifier tube 252 and this rectied signalvoltage is ltered and smoothed by the resistance-condenser lter 248,249, 250. over A. V. C. conductor'H through resistors 241 `and 235 tothe respective control grids of tubes .25| and 236. Thus when a drop inlevel occurs,

.the voltage rectified by tube 252 decreases, with a similar decrease innegative grid bias of tubes 236 and 25| and the gain is therebyincreased to bring the voltage back approximately to its formermagnitude. In this A. V. C. arrangement, the A. V. C. signal istherefore controlled not by The filter used in practice passes bethelevel of the received radio carrier voltage but [by the dem-odulatedsignal voltage. "I'hus, even though the carrier remains constant and thesideband signals fade, the demodulated signal will remain constant.

The other plate circuit of -tube 264 feeds the power limiter tube 21|.Tube 21| perfonms several functions. First the action of the A. V. C.circuit above described is not instantaneous because of theresistance-capacity filter and therefore on a sucl'den fade of thereceived radio signal, the latter will drop for a short period.Secondly, the A. V. C. is not perfect and should the signal fade to theorder of db., the A. V. C. will hold the fade down to only 3 or 4 dfb. Ihave found that this level variation is still too great for the bestresults in facsimile or picture. transmission. Tube 21| takes care ofboth these conditions by amplifying the signal impressed thereon bytransformer 269 up to a predetermined value and then as the signalincreases further the output remains constant over a very wide range.The tube 21| is preferably connected in balanced or push-pull relationto the secondary winding of transformer 269, and the controlgrids 21|a,21|b are at zero bias with respect to the associated cathodes '21|c,21|d. Consequently, as the grids swing positively, grid current flowsthrough the resistance 210. This automatically applies a bias to thetube 21| which bias builds up substantially in proportion to the Therectied voltage is then applied l input voltage, thus holding theoutput-substantially constant. Preferably, a resistance 213 is providedin the common plate lead to make the characteristics of the tube 21|linear up to a certain point and then to cause it suddenly to flattenout. This is probably due to the fact that as the grid voltage increasespositively, the bias built up, in resistor 210 does not increase byquite the same amount, but resistor 213 serves to lower the platevoltage as the plate current increases. The loutput of tube 21| is asubstantially square wave wherein substantially only odd harmonics Iarepresent as a result of .the balanced or pushpull connection. These oddharmonics must be eliminated before passing the signal to the convertercircuit, the elimination being effected by filter 214 which is alow-pass or preferably a The signal is then applied to the convertercircuit wherein the frequency changes in the signal are converted toamplitude changes. The control elements of this converter consist ofresistor 28|, condenser 282 and inductance 283'connected to the platecircuit of amplifier tube 216. The condenser 282 and inductance 283 aredesigned so asto be series resonant at some frequency below the lowestsignal frequency, which has been assumed to be 1800 cycles. In order tomake the resultant voltage applied to the control grid of amplifier tube281 linear with respect to frequency, the impedance of condenser282-inductance 283 combination must be low compared to resistance 28|even at the highest frequency passed. The series resonant frequency ofv282 and 283 is placed at the lower frequency attenuation peak ofbandpass filter 214, for example at approximately 1380 C. P. S. This isdone because at frequencies less than the said series resonantfrequencies, the voltage will rising to too greatl a value.

again increase and will reach a maximum vat very low frequencies, withthe result that any noise being received at frequencies less than thesaid l lowest signal frequency transmitted, e. g. 1800 C. P. S., thegreater will be the change of voltage with change of frequency, thusgiving a better response ratio of signal-to-noise. Another resistor 2'84is provided having about onetenth of the value of resistor 28`i and itserves to prevent low frequency noise that the filter -214 may not havecompletely eliminated, from This resistor 284 must also be high comparedto the maximum impedance of 282-283, otherwise an undesirable phaseshift will result.

The converted signal is'then applied to an amplifier tube 281 which maybe of any suitable type preferably a type 6C8 upon whose main controlgrid 281e the converted signal is impressed. The output of tube 281 isfurther coupled in balanced relation to another ampli- In order that theproper range of tone values may-be employed at the receiver R, meansareprovided for obtaining zero output of the locali ly generated 1800cycle carrier from tube 301 to correspond with zero voltage on thecontrolgrid 50a of tube 50 at the transmitter. This is accomplished byapplying a fixed Anegative bias on the plates 3I8c, 3|8d of themodulator tube 3|8 as indicated schematically by the bias source 350.By-means of thepotentiometer 215, the gain of tubes 216, 281, 30| and302, are then increased until the voltage developed across reer stagecomprising tubes 38| and 302 connected ,in push-pull relation wherebythe signal level is raised to the desired value. It will be understoodof course that any well-known linear amplifier may be used foramplifying the converted signal.

The amplified output of tubes H, 302, is then rectified in a suitablerectifier 224 and the rectified 'output is passed through a low-passfilter 304. This'fllter attenuates all frequencies above the lowesttransmission frequency, e. g. 1800 C. P. S. leaving only the demodulatedvarying amplitude signal corresponding to the original photo-electriccellsignal as delivered at the machine T (Fig. 1). The variableamplitude signal from filter A304 can `be applied directly to anyjwell-known picture reproducing machine. However, in the event that themachine R is of a type which is operated on a modulated audio frequency,for example an 1800 cycle tone, the

signal from filter 304 is used to modulate a locally generated 1800cycle source. For this `purpose, there is an oscillator tube 3 01 whichis provided with circuit connections 308, 309, 310, 3II, whereby asustained 1800 cycle tone is generated. This 1800 cycle tone or currentis then impressed in balanced relation on the control-grids 3I8a, 3|8bof a modulator tube 3|8. The plates 3I8c, 3l8d, are supplied with platepotential from the output of filter 304 which is connected in balancedrelation to the primary of coupling transformer 320. As a result, thereis impressed upon the balanced amplifier Atube 322, an 1800 cyclecurrent modulated in accordance with the signals from filter` 304. Ifdesired,l the outputof the amplifier of tube 322 may be passedthrough afilter 325 having a band-pass width suitable for passing the 1800 cyclelwave with the necessary sidebands. The modulated 1800 cycle signal maythen be passed through a suitable resistorv pad 321 to 33! and thence tothe facsimile reproducing machine R. It will be understood of coursethat the facsimile transmitting machine T and the facsimile reproducingmachine R are synchronized by any method well-known in the facsimileart.

sistor `305 is equal to the negative bias from the source 350. Thus, anet voltage of zero is applied to the plates of tube 3I8 which is thethreshold point or zero voltage point.v Under the above assumption of aminimum signal frequency of 1800 cycles, this zero voltage correspondstherefore to this lower signal frequency, consequently any change in thefrequency applied to tube 216 will give a corresponding 1800 cycleoutput from the modulator 3I8.

. In adjusting the receiver of Figs. 3 and 4, the average input isadjusted to normal by the meter 226- and the level of the receivedsignal Ais increased by potentiometer 215 until the voltvoltage to beimpressed on the plate of tube M0 resulting in a certain level of thelocal ,1800 cycle oscillations applied to tube 322. This level isadjusted to normal by increasing the gain by means of potentiometer 32|.After these preliminary adjustments at the transmitter and receiver, theentire system is in readiness for the transmission of the subject matterwhich may be a picture, written matter or any visual display consistingvariations of tone or shade values.

While in the foregoing description reference has been made to thetransmission of a picture by means of frequency modulations between 1800C. P. S. and 3000 C. P. S., it will be understood that the invention isnot necessarily limited thereto. However, I have foundv from actualtests and demonstrations that the above lshift of from 1800 to 3000 C.P. S. is more than sumcient to transmit all the frequencies present inmodern picture or facsimile transmitters. On the other hand, I havetransmitted with good results with the equipment described, using afrequency shift of from 1200 to 1800 cycles. Furthermore, whileparticular apparatus and parts have been described, it will beunderstood that -T by a voice frequency generator and by replacing thereproducer R by a voice frequency reproducer. With a higher range offrequencies such as between 5000 and 7500 C. P. S., or even .thereceiver RR. may be of the diversity type.

r5000 and'6500 C. P. S., frequencies up to 4500 C. P. S. can be passed.The invention is not limited to facsimile transmission or to ordinaryvoice 'frequency telephony. The system is well suited for secret radiotransmission because the original voice signals that may be impressed bythe transmitter T are radiated from the radio transmitter RT in anunintelligible form. and in order that they may be reproduced, areceiver such as shown in Fig. 2 and adjusted and correlatedwith thetransmitter of Fig. 1, must be employed. I have also found that a systemas described when used in facsimile or voice communication eliminates toa substantial extent. well-known echo effects. As a result of using thesystem as disclosed, the following among other results have beenobtained. Y

1. Unattenuated frequency shifts up to the limit of the low-pass filter45.

2. Constant amplitude of the oscillator 86 with resultant constantamplitude ofthe beat frequency thereby introducing no frequencycomponent of the picture machine photo-electric cell voltage.

3. Linear variation of frequency with voltage over a very wide range.

4. Independency of the frequency with respect to the power supplyvoltages for tubes 65 and 86.

5. The elimination of all harmonic distortion by using the variouslow-pass and band-pass filters such as |01, 206 and 214 and theelimination of all noise below the actual frequency range used for`signaling.

6. The automatic volume control is rendered more effective by beingcontrolled by the demodulated signal in tube 25| ygiving constantamplitude to the converted signal applied to tube 216.

7. The supplementing of the automatic volume control by the limiter tube21| and filter 214 whereby all changes in level which escape theautomatic volume control circuit are ironed out, thus supplying anundistorted signal at perfectly constant level to the succeedingcircuit.

8. The provision of a converter circuit asso- 45 ciated with tube 216whereby frequency changes are made to amplitude changes with perfectlinearity and with high ratio of signal-to-ncise.

It will be understood of course that while one particular form of levelcontrol has been dis- -5o closed in connection with the tubes 236 and25|, any other well-known form may be employed and in addition ifdesired, yanother A. V. C. signal may be derived from the radio carrierand if desired,-

The A.'V. C. voltage derived as shown may be applied to control the gainof the receiver itself,

being applied to all controlled tubes in the re# ceiver, or to some ofthem, leaving the radiocarrier-derived A. V. C. on the others.

While in the foregoing, the frequency conversion at the transmitter iseffected by a fixed frequency oscillator 86 and a variable frequencyoscillator 65, a pair of variable frequency oscillators may be employed.Suchan arrangement 6 is shown in Fig. 6 wherein the parts shown betweenthe dot-dash lines may be substituted for the part of Fig. 2 between thedot-dash lines. In Fig. 6 the left-hand rectangle representsdiagrammatically the transmitting equipment shown in detail in Fig. l;while the right-hand rectangle of Fig.Y 6 represents diagrammaticallythe part of Fig. 2 to the right of the dot-dash line. In the embodimentof Eig. 6 the signal conductors, J, J I are connected not only to thetube\ 6 6 as described in connection with Fig. 2, but are also connectedto a similar tube 402 so that the amplitude variations on conductors J,J l. result in simultaneous shifting inopposite directions of thefrequency of the oscillators and 88. For this purpose the resistance 41is connected in balanced relation across the control-grid of tube I0 andthe control-grid of tube 462. the connection to the latter gridincluding a potential source 40| poled as shown and having a potentialequal tc the peak voltage swing applied to conductors J. JI. The plateoi tube 60 is coupled through condenser 84 to a tunable oscillatorycircuit such as that represented by |28, Il, 68, 6| `(Flg. 2). Likewisethe plate of tube 402 is coupled through a similar condenser 84a to atunable oscillatory circuit 16, 16, associated with the oscillator tube86. If desired, the tuned circuit 18, 16, may be replaced by a circuitsimilar to that of Fig. 2 including elements |28, 68, 60 and 6|. 'Ihearrangement of Fig. 6 has the advantage that the frequency shift for agiven amplitude variation on conductors J, J is doubled as compared withthe arrangement of Fig. 2. It also has the advantage kthat anynon-linearity cf frequency shift on one oscillator tube, e. g..tube 65is balanced by an equal and opposite non-linearity on the other tube 86.

Instead of employing a series condenserinductance combination 282, 283(Fig. 4) for inverting the frequency modulations to amplitudevariations, an arrangement such as shown in Fig. 4A may be employedwherein the parts corresponding to those of Fig. 4 bear the same nu`-merals. In this modification, the condenser 218 is designed to be ofhigh impedance compared to resistance 284. The voltages developed acrossresistance 284 as a result of the frequencies impressed onv tube 216,are used to control the amplitude in the output of tube 281. Instead ofusing a series resistance-condenser, a series resistance-inductancecombination may be employed, this modification being shown in Fig. 4Bwherein the impedance of inductanee 283 is low compared to resistance28|. While the embodiment of Figs. 4A and 4B each give zero voltage atzero frequency when operating between 1800 and 3000 C. P. S., onlyapproximately 67 percent voltage change is produced; whereas with theseries resonant circuit 282, 283 of Fig. 4, when this combination isresonant at 1400 C. P. S., over the same frequency range of 1800 to 3000C. P. S., there is obtained a 300 percent voltage change. Variouschanges and modifications may be made in the disclosure withoutdeparting from the spirit and scope of the invention. Thus, if undercertain conditionsof operation, for example at very high speeds oftransmission, undesirable phase shifts occur in the various filtersdescribed, these phase shifts may be compensated for by connecting incircuit with one or more of the filters suitable phase equalizers.However, it has been found that at ordinary commercial transmissionspeeds, such equalizers are not necessary.

What I claim is:

l. The method of signaling which includes the steps of generatingsignals of different amplitudes representing different signal conditionsto .be transmitted, said signals being generated within a frequencyrange which is lower than that required for radiation through space,converting said signals into corresponding variableto a frequencyspectrum of substantially the same order as said first-mentionedsignals, and further limiting thee band of said converted signals sothat the uppermost frequency is less than the second harmonic: ofthelowermost frequency.

2. The methodiof signaling which includes generating variable amplitudesignals in the audio frequency range. impressing said signals on afrequency' modulator to produce7 a frequency4 range, adiusting saidmodulator so that the frequencyl range thereof corresponding to the'lower and upperf limits ofthe said signal amplitude is such; that: theuppermost converted audio frequency' is; less than the second harmonicof the lowermost converted audio frequency, passing said convertedfrequencies through a bandpass, filter to cut-off second harmonics ofany transmitted frequency, and modulating a high frequency carrier waveby the output of said filter. .i

3. The method of reducing the effects of selective fading intransmission over a radio channel and the like which comprises,converting variable amplitude signals in the audio frequency range intocorresponding variable frequency signais also in the audio frequencyrange, limiting the frequency range of the converted signals so that theuppermost audio frequency is always less than the second harmonic of thelowermost audio frequency, and applying the converted frequencies tomodulate a radio carrier wave.

4.'In a signaling system, means to generate variable amplitude audiofrequency signals, means to convert said signals into variable frequencysignals of substantially uniform amplitude but confined within the audiofrequency range, means to limit the converted signals to an audiofrequency range such that the uppermost. frequency most frequency, andmeans to modulate a high frequency carrier wave by said convertedfrequencies.

5. A signal transmission system wherein the signals to be transmittedare of different amplitudes corresponding to different signalconditions, means to convert said signals into corresponding beatfrequency signals with the beat frequencies confined to an audiofrequency range, and lter means upon whichA said beat frequencies areimpressed, said filter passing only frequenciesl below the secondharmonic of the lowermost beat frequencies and means to modulate a highfrequency carrier wave by said passed frequencies.

6. A signal transmission system comprising means to generate signals inthe form of a modulated alternating current of audio frequency withauido frequency signal modulations, means to detect said modulations,means to impress only said detected modulations on the grid of agridcontrolled electron tube to vary thereby the effective shuntcapacitance of the oscillatory circuit of an oscillator generator tovary the frequency of said oscillator without substantially affectingthe amplitude of the generated oscillations, means to transmit saidvariable frequencies over a high frequency carrier channel by amplitudemodulation of a high frequency carrier by said variable frequencies,said grid-controlled tube and said oscillator being provided withadjustable means to correlate the amplitude of the said impresseddetected modulations with the frequency variations of the oscillator sothat the upper limit of the converted frequency range is less than thesecond harmonic of the lower limit of said range.

7. A signal transmitter comprising `means'to generate signals' in theform of a modulated alternating current, means to detect saidmodulaltions.v means to convert the amplitude of said modulations into.corresponding frequencies, means to determine the lower frequency linutof said converted frequencies including a mixing network, and means toimpress on said network a part of the said modulated alternating currentand a part of said converted frequencies for comparison.

8. The method of transmitting pictures or the like over a transmissionchannel which tends to introduce harmonic distortion which methodcomprises, generating picturesignals in the form of afrequency-modulated audio frequency carrier, limiting the frequencyspectrum of the modulated carrier.l so that the uppermost frequency isless than the second harmonic of the lowermost frequency, transmittingsaid frequency modulated carrier over said channel, receiving anddetecting said frequency modulations, filtering out all frequenciesoutside the range corresponding to the said spectrum as limited at thetransmitter, and applying said spectrum to control a picture reproducer.

9. The method of transmitting pictures or the like over a transmissionchannel which tends to introduce harmonic `distortion which comprises,frequency-modulating an audio frequency carrier in accordance withpicture signals to produce a frequency spectrum wherein the lowermostfrequency represents one picture shade and the uppermost frequencyrepresents substantially the opposite shade, receiving andfrequency-demodulating said spectrum to convert it into correspondingamplitude modulated signals and limiting the spectrum both at thetransmitter and at the reeciver so that prior to conversion into saidamplitude modulations at the receiver the uppermost frequency is lessthan the second harmonic of the lowermost frequency.

10. The method of transmitting pictures or the like over an existingradio transmission system of the amplitude-modulated high frequencycarrier wave type which method comprises, scanning the picture toproduce signals in the form of an audio frequency carrieramplitude-modulated by audio frequency signals, amplifying theamplitude-modulated carrier and rectifying it to derive the saidmodulations, converting said modulations into a frequency-modulatedcarrier within the audio frequency range, limiting the frequency band ofthe frequency spectrum of the frequency-modulated carrier to one whereinthe vuppermost frequency is less than the second harcarrier wave havingamplitude modulations corresponding tc the picture shades, amplifyingthe frequency carrier, limiting the spectrum of the convertedfrequencies so that the uppermost frequency represents one degree ofshade and the lowermost frequency represents substantially the oppositedegree of shade and with the uppermost frequency lower. than the secondharmonic of the lowermost frequency, amplitude modulating 'a radiocarrier wave by said converted frequencies, receiving and detecting fromsaid radio wave the said converted frequencies, filtering out from saidconverted frequencies all frequencies exceeding those within the saidspectrum.- andapplying the passed frequencies to control a plcturereproducer.

12. A signal transmitter comprising means to generate signals in theform of a modulated alternating current of audio frequency with-audiofrequency modulations, means' to detect said modulations, means toimpress only said detected modulations on the grid of a grid-controlledelec tron tube to vary thereby theeffective shuntcapacitance of theoscillatory circuit of an oscillator generator to vary the frequency ofsaid oscillator without substantially aecting the amplitude of thegenerated oscillations, means to transmit said variable frequency over ahigh frequency carrier channel by amplitude modulation of the highfrequency carrier by said variable frequencies, and manual adjustingmeans for adjusting the lower limit of the frequency from saidoscillator to a predetermined value -when no detected modulations areimpressed on said tube and for adjusting the maximum level ofthedetected modulations so that the upper limit of the frequencies ,fromsaid oscillator is less than the second harmonic of the lower limit ofsaid fre- I quencies. a

13. In a facsimile system, afacsimile transmittervhaving means toconvert the tone values of a picture or the like into afrequency-modulated audio frequency carrler wherein the frequencyspectrum is limited so that the uppermost frequency corresponding to oneextreme range of tone value is less than the second harmonic of thelowermost frequency corresponding to the opposite range of tone value,means to transmit said carrier over a transmission channel which vissubject to harmonic distortion, means to receive said carrier and toeliminate therefrom all l.parasitic frequencies which are a second orhigher "harmonic of the lowermost frequency of said spectrum. and afacsimile reproducer controlled by CERTIFICATE oFlcoRREcTIoN,

'Patent No" 2"`2 99,957 October 27,v

ROBERT n. sPBAGu-E.-

`It'.lsfhe'aby certified than: error appears in the prntedfspeeiifiotionof the abovelnumbered patent requiring oyrreotion'as follows: Page",'first co1m,",11.nef9, clamZ, Vafter "a" second 'occt'arence5-l insertfrequency mogulated'ucarier also -in the audio-F; and that the .said'Lette'rs-.Pstent should'be read'A with this' correction therein thatthe samefm'ay'confong to theA v mme 'o'f ,the case 'chepate'nt office.

Signed'end seeled.' this `'163:11 'day Cif-November, A. D. 1.9145..

Henry Van Arsdale, (Seal) Acting Commissioner'- of Patents.

